Biosensors are devices for sensing and detecting biomolecules. The biosensors operate on the basis of electronic, electrochemical, optical, or mechanical detection principles. Biosensors including transistors are sensors that electrically sense charges, photons, or mechanical properties of bio-entities or biomolecules. The detection can be performed by detecting the bio-entities or biomolecules themselves, or through interaction and reaction between specified reactants and bio-entities/biomolecules. Such biosensors can be manufactured using semiconductor processes, can quickly convert electric signals, and can be easily applied to integrated circuits (ICs) and microelectromechanical systems (MEMS).